Expose the full set of outbound HTLCs from a `ChannelMonitor`

Expose the full set of outbound HTLCs from a `ChannelMonitor`

This expands the outbound-HTLC-listing support in `ChannelMonitor`
to include not only the set of outbound HTLCs which have not yet
been resolved but to also include the full set of HTLCs which the
`ChannelMonitor` is currently able to to or has already finalized.

This will be used in the next commit to fail-back HTLCs which were
removed from a channel in the ChannelMonitor but not in a Channel.
Using the existing `get_pending_outbound_htlcs` for this purpose is
subtly broken - if the channel is already closed, an HTLC fail may
have completed on chain and is no longer "pending" to the monitor,
but the fail event is still in the monitor waiting to be handed
back to the `ChannelMonitor` when polled.